This invention relates to cold-curing foundry sand binders, i.e., binders which cure at about ambient temperature. The binders for manufacturing foundry cores and molds of the type capable of self-curing at ambient temperatures are classified into inorganic binders, which have acquired traditional commercial acceptance, and organic binders, which have been gaining in commercial acceptance in recent years.
The organic binders generally cure by polycondensation of the resin accompanied by the simultaneous formation of water. The water formed during the polycondensation reaction impedes the progress of the cure, and consequently, the rate of cure differs widely between the surface and the inner part of the core or mold. Therefore, when a large core or mold is manufactured, much time is required before the core or mold is ready for removal from the pattern. The use of a strong acid, such as sulfuric acid, has been proposed as an effective means of increasing the rate of cure. This method, however, is not suitable for the manufacture of large cores or molds due to a decrease in the useful life of the resin composite. Moreover, this method requires the use of additional procedures, such as, for example, applying heat to the silica sand in order to maintain a fixed temperature throughout the year despite seasonal variations in ambient temperatures.
A binder for the manufacture of cores or molds which overcomes the problem of nonuniform cure of the core or mold, colloquially referred to as the "Linocure Process," has been commercially introduced into Japan. According to common practice, the binder for the manufacture of cores and molds comprises an oil-modified alkyd resin, a polyisocyanate, and a curing accelerator. The core or mold is produced by mixing silica sand with these components to form a silica sand-binder mixture, and molding the mixture into a pattern.
U.S. Pat. No. 3,255,500, to Engel et al., discloses admixtures of oil-modified alkyd resins and polyisocyanates as no-bake binders for foundry compositions. Engel et al. discloses various oils as components of the oil-modified alkyd resin employed therein, including tall oil, linseed oil, and mixtures thereof. Engel et al. do not, however, recognize the potentiated effects occurring through the selection of specific proportions of ingredients which are essential to the instant invention. More particularly, the proportions of tall oil fatty acid to linseed oil of the instant invention, when employed as the oil component of the oil-modified alkyd resin, provide significantly faster curing of the mold and generate considerably less objectionable odor than other proportions included within the broad sweep of the disclosed combination of ingredients.
As is true for conventional binders, this process possesses several advantages including good collapsibility of the cores or molds after casting, reusability of recovered sand, and fewer process steps in the finishing process. Furthermore, this process is more advantageous than methods utilizing furfuryl alcohol-modified resins in the sense that removal of the core or mold from the pattern can be attained within a shorter period of time. This characteristic of permitting a relatively short pattern residence time has caused this process to rapidly gain in commercial acceptance in recent years.